It is known to include a cushion sub-assembly below the drill head of a drilling rig and through which the turning force of the drill head is transmitted to the uppermost drill pipe section to thereby rotate the drill string within the bore being drilled. Such cushion sub-assemblies have taken a number of different forms, the design of which takes into account the features of the drilling operation in which the cushion sub-assembly will be used.
For example, it is common in a drilling operation to force the drilling bit into engagement with the bottom of the bore and to achieve the cutting action due to rotational motion only of the drill string and drill bit. A cushion sub-assembly designed for this type of drilling must be capable of absorbing a significant portion of both the torsional and axial vibrations resulting from the cutting action of the bit.
In downhole hammer drilling, compressed fluid is transmitted to a hammer means at the bit location and this results in additional axial vibration. Cushion sub-assemblies used for this type of drilling have therefore needed to be capable of transmitting a drilling torque as well, despite the cushion sub-assembly being primarily designed to prevent the transmission of high axial vibration forces to the drilling head. Existing cushion sub-assemblies are not satisfactory as vibration is still carried through to the rotary head.
A further problem with existing cushion sub-assemblies of this type is their tendency to wear very quickly. The internal components of the cushion sub-assembly, particularly those in contact with the reciprocating piston, require regular maintenance, refurbishment, and eventual replacement. Refurbishment of existing cushion sub-assemblies is time consuming and expensive due to the number of parts requiring maintenance. Accordingly, there is a clear commercial benefit in having a cushion sub-assembly that boasts an increased lifespan, and which is designed to be quick, simple and inexpensive to refurbish.